如何从Java中分配给它们的值中检索枚举常量？

Question

I am trying to get Enum constant through value assigned to it but don't know if there is any built in API to do this. My enum looks like this :

public enum VideoBandwidth {

    VIDEO_BW_AUTO(-1),
    VIDEO_BW_OFF(0),
    VIDEO_BW_2_MBPS(2000000),
    VIDEO_BW_500_KBPS(500000),
    VIDEO_BW_250_KBPS(250000);

    private final int bandwidth;

    private VideoBandwidth (final int value) {
        bandwidth = value;
    }

    public int getValue() {
        return bandwidth;
    }
}

How do I get enum constant VIDEO_BW_2_MBPS through value "2000000" assigned to it ? I understand that if values are sequential like 0,1,2,3, I can use VideoBandwidth.values()[index] but how do I get the constant in case when values cannot be used as Index ?

Answer 1

public static VideoBandwidth withValue(int value) {
    for (VideoBandwidth v : values()) {
        if (v.bandwidth == value) {
             return v;
        }
    }
    throw new IllegalArgumentException("no VideoBandwidth with value " + value);
}

Of course, you can also store the enum values in an internal Map, for example, if you want to avoid the iteration and the array creation.

Answer 2

Implement your own method that iterate over all the constants and returns the appropriate one or null /some exception.

public VideoBandwidth valueOf(int bandwidth) {
    for (VideoBandwidth videoBandwidth : values()) {
        if (videoBandwidth.bandwidth == bandwidth)
            return videoBandwidth;
    }
    throw new RuntimeException();
}

Answer 3

Iterate just once! define a static Map and fill it in a static block at load time.

final static Map<Integer, VideoBandwidth> cache = new HashMap<>();
static {
    for(VideoBandwidth e: VideoBandwidth.values()) {
        cache.put(e.getValue(), e);
    }
}

public static VideoBandwidth fromValue(int value) {
    VideoBandwidth videoBandwidth = cache.get(value);
    if(videoBandwidth == null) {
        throw new RuntimeException("No such enum for value: " + value);
    }
    return videoBandwidth;
}

Answer 4

Use a map:

public enum VideoBandwidth {

    VIDEO_BW_AUTO(-1),
    VIDEO_BW_OFF(0),
    VIDEO_BW_2_MBPS(2000000),
    VIDEO_BW_500_KBPS(500000),
    VIDEO_BW_250_KBPS(250000);

    private final int bandwidth;
    private static final Map<Integer, VideoBandwidth> map = new HashMap<Integer, VideoBandwidth>();

    private VideoBandwidth (final int value) {
        bandwidth = value;
        map.put(value, this);
    }

    public int getValue() {
        return bandwidth;
    }

    public static VideoBandwidth valueOf(int bandWidth) {
        return map.get(bandWidth);
    }
}